Vitreous coating materials, commonly referred to as glazes or enamels, are used on many ceramics and other brittle and low thermal conductivity materials for aesthetic and functional purposes. Ceramic whitewares are coated with glazes for decorative and sanitary purposes. Conductive glazes are used on ceramic electrical insulators to remove charges. Glazes are also used in ceramic electronic packages for hermetic sealing and bonding. Enameled cast iron is coated with porcelain enamels for decorative and functional purposes.
Glazed ceramic manufacturing processes often yield a certain percentage of product that have surface defects. These defects can detract from the appearance of the article and create functional problems that render the article unusable. Consequently, methods have been developed for repairing certain types of surface defects in glazed ceramics. One such repair method involves enlarging the hole by drilling, filling the hole with a glaze material similar to the original, and re-firing the entire part at maximum temperatures near 1200.degree. C. for many hours. Unfortunately, new flaws may initiate at secondary sites during re-firing, thus, frustrating the repair efforts. Moreover, this method is expensive, energy intensive, and time consuming.
Another repair method considered for use in repairing glazed ceramics is laser welding. Laser welding methods typically employ a carbon dioxide (CO.sub.2) laser operating at a wavelength of 10.6 .mu.m. Such lasers are used because they are stable and capable of delivering high, readily absorbed output power such that temperatures of 1800.degree. C.-1900.degree. C. over a few square millimeter area can be rapidly achieved to fuse refractory materials within a few seconds. The rapid heating can also produce undesirable cracking within and near the irradiated region due to the brittle and low thermal conductive nature of ceramics which permit high thermal gradients and high stresses to develop.
Laser induced thermal stress problems have been addressed in U.S. Pat. No. 5,427,825 which discloses a method for localized glazing of ceramics that maintains a non-destructive level of thermal stresses within the irradiated region by appropriate control of the laser's energy. This is accomplished by applying a fusing radiant energy to a fusion zone on the surface of the glazed ceramic article and annealing the fusion zone by applying annealing radiant energy to an annealing zone on the surface of the article which encompasses the fusion zone. The annealing radiant energy limits the thermal stress to a level during cooling which is less than the fracturing stress of the ceramic material, so that cooling can be accomplished without inducing cracks in the surface.
The glazes coating the ceramics described in U.S. Pat. No. 5,427,825 typically have melting temperatures near 1200.degree. C., and coefficients of thermal expansion of 6-7.times.10.sup.-6 /.degree. C. The engineering of these two parameters has been found to be critical in laser fusion performance, particularly in laser sealing of vitreous coatings over brittle substrates. The high melting temperature requires the absorption of considerable energy to fuse the coating to the substrate. Consequently, the non-destructively high thermal gradients and the thermal expansions in the range noted, can still increase thermally-induced strains beyond critical limits, thus resulting in permanent damage to the coating and the substrate.
Accordingly, coating materials are needed which have coefficients of thermal expansion and glass transition temperatures sufficiently lowered to a point where the strain induced by a rapid change in temperature during localized thermal processing, remains below the elastic strain limit. There is also a need for filler materials that can be used for repairing deep defects via localized thermal processing.